The term “inhibitor switch” is hereinafter used to mean “a sensor for detecting a shift range designated by a driver”, and therefore should be understood to have that meaning.
An automatic transmission for a vehicle automatically engages an appropriate shift-speed on the basis of parameters related to driving states of a vehicle, such as vehicle speed and throttle opening. An automatic transmission for a vehicle is usually provided with a shift lever by which a driver can designate a desired shift range such as “P”, “R”, “N”, “D”, “2”, and “L”.
A manual valve of the automatic transmission is repositioned according to operation of the shift lever, and accordingly, the automatic transmission forms hydraulic control lines appropriate for the position of the shift lever. The position of the manual valve is detected by an inhibitor switch, and the detected position is transmitted to a transmission control unit (TCU) that controls overall operation of the transmission. Accordingly, the TCU controls hydraulic pressure supply to engage an appropriate shift-speed at the shift range designated by the driver.
In the case of malfunctioning of the inhibitor switch, such as a case in which no signal is generated at the inhibitor switch due to, e.g., broken circuits, the TCU controls the automatic transmission in a mode that can keep the vehicle running. Such a control mode is frequently called a Limp-Home mode.
According to the prior art regarding such a Limp-Home mode, the TCU usually controls the automatic transmission under the supposition that the last received signal from the inhibitor switch is still being received.
According to such a control method in a Limp-Home mode of the prior art, the vehicle can keep running at a fixed shift-speed, but shifting operation among a plurality of forward shift-speeds on the basis of vehicle parameters such as vehicle speed and throttle opening is disabled.
For example, in the case that the inhibitor switch malfunctions while the shift lever is moved from neutral N range to drive D range, the last received signal from the inhibitor switch indicates the N range, while the manual valve is repositioned to the drive D range.
In this case, hydraulic pressure can be supplied to friction elements for a specific shift-speed and therefore the specific speed may be engaged, since the manual valve is actually positioned to the drive D range. However, shifting operation from the specific shift-speed to another shift-speed is disabled, because the TCU regards the current position of the shift lever as fixed to the N range and accordingly it does not generate signals for shifting. Accordingly the vehicle can only run at the specific shift-speed.
In the case that a vehicle can only run at a fixed shift-speed in such a Limp-Home mode, the transmission cannot sufficiently provide appropriate operation for a wide range of vehicle speeds. For example, when the transmission is fixed to a lower gear such as a second forward speed, the vehicle cannot run at high speed since the engine speed is limited. When the transmission is fixed to a higher gear such as a fourth forward speed, the vehicle suffers from lack of driving power in a low vehicle speed.
The information disclosed in this Background of the Invention section is only for enhancement of understanding of the background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art that is already known to a person skilled in the art.